[camera_web] Release the camera stream used to request video and audio permissions

[bselwe]

The camera stream used to request video and audio permissions is now released before accessing available camera devices. This solves a cameraNotReadable error thrown when requesting a camera stream that is already in use.

Fixes flutter/flutter#90040.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[bselwe]

@ditman

[ditman]

@googlebot I consent.

[ditman]

We had discussed this a couple of weeks ago. LGTM!

[ditman]

(Making this PR a NEXT release, so it can be published alongside #4210)